In the technical field of coal pyrolysis, the medium-low temperature destructive distillation of coal generally employs an internal-heating shaft furnace. Internal-heating shaft furnaces are widely used because of its simple structure and low cost. However, the greatest disadvantage of the internal-heating shaft furnaces is that coal gas is mixed with the flame gas, wherein the coal gas has a low heat value, except the use of part of the coal gas for pyrolysis itself, most of the coal gas, if not being used in a lime kiln or power generation for co-production, will only be combusted and then emitted into the air, which not only causes energy waste, but also causes great environmental pollution problems. Relatively, fewer external-heating furnaces are used. Additionally, a common weakness in the prior art is that the combustion mechanism or the heating mechanism is disposed in the kiln body. This weakness is reflected in the following aspects, i.e., the combustion condition can hardly be observed, the combustion mechanism is difficult to repair and maintain, and it is difficult to use pulverized coal as the combustion medium, otherwise, the heat dissipation pipelines will be blocked easily.